1. Field of the Invention
The present invention relates to a power source apparatus, and particularly, to a controller in the power source apparatus.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a power source apparatus according to a related art. In FIG. 1, an input power source Vin is a DC power source whose both ends are connected to a first series circuit including a step-up reactor L1, a switching element Q1 made of a MOSFET, and a current detecting resistor R1. Both ends of the series-connected switching element Q1 and current detecting resistor R1 are connected to a rectifying-smoothing circuit including a rectifying element D1 and a smoothing capacitor C1. The step-up reactor L1, switching element Q1, current detecting resistor R1, and rectifying element D1 constitute a first converter.
Both ends of the input power source Vin are also connected to a second series circuit including a step-up reactor L2, a switching element Q2 made of a MOSFET, and a current detecting resistor R2. Both ends of the series-connected switching element Q2 and current detecting resistor R2 are connected to a rectifying-smoothing circuit including a rectifying element D2 and the smoothing capacitor C1. The step-up reactor L2, switching element Q2, current detecting resistor R2, and rectifying element D2 constitute a second converter.
Both ends of the smoothing capacitor C1 are connected to a series circuit including resistors R3 and R4. This series circuit forms an output voltage detector that detects a voltage at a connection point of the resistors R3 and R4 and provides an output voltage signal VFB.
According to the output voltage signal VFB, a controller 13b turns on/off gates of the switching elements Q1 and Q2 to control a voltage across the smoothing capacitor C1 to a constant value.
FIG. 2 illustrates operating waveforms at various parts of the power source apparatus of FIG. 1. In FIG. 2, a signal g1 is a drive signal for the switching element Q1, g2 is a drive signal for the switching element Q2, Q1i is a drain current of the switching element Q1, Q2i is a drain current of the switching element Q2, OCP1 is an overcurrent detection signal of the switching element Q1, OCP2 is an overcurrent detection signal of the switching element Q2, IL1 is a current passing through the step-up reactor L1, IL2 is a current passing through the step-up reactor L2, and Iin is a current passing through the input power source Vin.
Operation of the power source apparatus of FIG. 1 will be explained with reference to FIG. 2.
The controller 13b outputs the drive signals g1 and g2 to drive the switching elements Q1 and Q2. When the switching element Q1 is ON, the current Q1i passes in a clockwise direction through a path extending along Vin, L1, Q1, R1, and Vin, to accumulate flux energy in the step-up reactor L1. When the switching element Q1 changes from ON to OFF, the flux energy accumulated in the step-up reactor L1 causes a current passing in a clock wise direction through a path extending along Vin, L1, D1, C1, and Vin, to charge the smoothing capacitor C1.
When the switching element Q2 is ON, the current Q2i passes in a clockwise direction through a path extending along Vin, L2, Q2, R2, and Vin, to accumulate flux energy in the step-up reactor L2. When the switching element Q2 changes from ON to OFF, the flux energy accumulated in the step-up reactor L2 causes a current passing in a clockwise direction through a path extending along Vin, L2, D2, C1, and Vin, to charge the smoothing capacitor C1.
The controller 13b controls the switching elements Q1 and Q2 so that a phase difference of 180 degrees is produced between the switching elements Q1 and Q2. This reduces current ripples of the input power source Vin and smoothing capacitor C1.
An overcurrent detector 11a includes comparators CP10 and CP11 and a reference voltage Vref. If a voltage across the current detecting resistor R1 becomes equal to or higher than the reference voltage Vref, the overcurrent detector 11a determines that the current detecting resistor R1 is passing an overcurrent and outputs the overcurrent detection signal OCP1 to the controller 13b. If a voltage across the current detecting resistor R2 becomes equal to or higher than the reference voltage Vref, the overcurrent detector 11a determines that an excess current is passing through the current detecting resistor R2 and outputs the overcurrent detection signal OCP2 to the controller 13b. 
Receiving the overcurrent detection signals OCP1 and OCP2 from the overcurrent detector 11a, the controller 13b stops the drive signals g1 and g2 to the gates of the switching elements Q1 and Q2, thereby limiting currents to the step-up reactors L1 and L2 within a predetermined range. This results in limiting currents to the switching elements Q1 and Q2 within a predetermined range.
The related art must arrange the current detecting resistor for each of the converters connected in parallel with each other. In addition, the related art must arrange, for each converter, the comparator in the overcurrent detector 11a. As results, the related art increases circuit scale and costs in proportion to the number of the parallel-connected converters.
FIG. 3 is a circuit diagram illustrating a power source apparatus according to another related art. The power source apparatus of FIG. 3 differs from the power source apparatus of FIG. 1 in that it connects a current detecting resistor R1 between a negative terminal of an input power source Vin, removes the current detecting resistor R2, and employs an overcurrent detector 11b. The remaining configuration of FIG. 3 is the same as that of FIG. 1.
FIGS. 4A to 4D illustrates operating waveforms of various parts of the power source apparatus of FIG. 3. In FIGS. 4A to 4D, OCP1 is an overcurrent detection signal related to currents passing through step-up reactors L1 and L2. The other reference marks depicted in FIGS. 4A to 4D are the same as those illustrated in FIG. 2. Operation of the various parts illustrated in FIGS. 4A to 4D is basically the same as that of FIG. 2.
The overcurrent detector 11b includes a comparator CP10 and a reference voltage Vref. If a voltage across the current detecting resistor R1 becomes equal to or lower than the reference voltage Vref, the overcurrent detector 11b determines that an overcurrent is passing through the current detecting resistor R1 and outputs the overcurrent detection signal OCP1 to a controller 13c. Receiving the overcurrent detection signal OCP1, the controller 13c stops drive signals g1 and g2 to switching elements Q1 and Q2.
Duty ratios of the drive signals g1 and g2 are controlled according to an input voltage, an output voltage, output power, and the like. If the ON duty ratio of each drive signal is 50% or smaller, the drive signals for the switching elements Q1 and Q2 do not overlap each other.
If the ON duty ratio of each drive signal exceeds 50% as illustrated in FIG. 4C, the drive signals for the switching elements Q1 and Q2 partly overlap. If an overcurrent is detected in the overlapping period, the controller 13c is unable to determine which of the drive signals must be stopped.
If both the switching elements Q1 and Q2 are stopped, a large difference occurs between the ON periods of the switching elements Q1 and Q2, to cause a large difference between currents passing through the switching elements Q1 and Q2 as illustrated in FIG. 4A.
There is another problem illustrated in FIG. 4B. A current value before the overcurrent protective operation is halved after the overcurrent protective operation. To cope with this problem, the phase difference between the switching elements Q1 and Q2 may be nullified at the start of the overcurrent protective operation, to improve the current reduction ratio to about 0.7 of the value before the overcurrent protective operation. The current reduction, however, is unable to eliminate.
A related art is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2007-195282.